Third stage of HTTP processing:
This method is called when the HTTP request has been completely received,
and the response is going to be generated. This method can again be called
from a different thread than the previous stages. It is allowed to spend
any necessary time to compute the response.

When the method returns, the request processing is finished. No more data
is allowed to be written to the output channel past this moment.

The method may raise Standard_response to generate one of the
standard messages.